Newton's Second Law establishes that all firearms, when fired, will recoil. Conservation of momentum requires that as a bullet is accelerated and propelled out the barrel of a gun, a force will be imparted in an opposite direction. This resultant force is equal to the rate of change of the bullet's momentum, and it is distributed among many elements in and on the firearm, such as the spent casing. Primarily, though, it acts within the chamber of a firearm as a recoil force. Recoil force causes the firearm to recoil or move backward quickly. “Felt recoil” is the force the shooter feels when the firearm moves backward into the shooter's hand, arm, and shoulder.
For some firearms, felt recoil can be very small. Some small-caliber handguns or rifles have little to no detectable felt recoil. Others, of course, have massive felt recoil. Prolonged use of such weapons is burdensome: they are difficult to control, rough on the body, and often need to be re-aimed each time after firing.
Various efforts have been made to reduce recoil and felt recoil. Some firearm systems use an open chamber to mitigate recoil. For example, recoilless rifles and some shoulder-mounted rocket launchers employ a barrel which is open at opposed ends. The projectile is held in the barrel until firing, at which point the combustion gases are exhausted out the open back of the barrel and the projectile is propelled out the front. Because the combustion gases do not impact any back wall of a chamber, little recoil force is applied to the weapon. As a result, the shooter experiences very little felt recoil.
Recoilless rifles fire very large artillery which would otherwise produce an uncontrollable or possibly injurious recoil force; the mitigation of their recoil is absolutely necessary. Most weapons, however, are not designed for the purpose of eliminating felt recoil. They fire sufficiently small projectiles that the felt recoil can be handled. Nevertheless, mitigation of recoil force and its deleterious effects in such weapons is still desirable, as it allows the shooter to fire more frequently, longer, and with greater accuracy.